A gunman behind the worst mass shooting in US history that left 50 people dead at an Orlando, Florida gay nightclub has reportedly been identified as Omar Saddiqui Mateen, a U.S. citizen of Afghan descent.

The early Sunday attack ended when police stormed the Pulse nightclub, fatally shooting the suspect and rescuing club-goers trapped inside. Another 53 people were wounded.

An FBI official said the mass shooting is being investigated as an "act of terrorism."

FBI spokesman Ron Hopper said at an Orlando news briefing it is premature to definitely classify the shootings as terrorism or a hate crime. That determination will come "once [investigators] have all the facts in place."

Orlando Mayor Buddy Dyer said Sunday that a state of emergency has been declared. Orlando Police Chief John Mina told reporters that the shootings are "a tragedy not only for the city, but for the entire nation."

Obama to address the nation

The White House said President Barack Obama will issue a statement on the shooting at 1:30 pm Washington time. The White House earlier said the president ordered the federal government to provide investigators with "any assistance necessary." It also said, "Our thoughts and prayers are with the families and loved ones of the victims."

On the gay club's Facebook page, a post around 2 a.m. gave early indication of the tragedy that was unfolding.

Why the club was targeted remains unclear. The shooting comes as many cities around the world celebrate June as Lesbian, Gay, Bisexual, and Transsexual (LGBT) Pride Month.

The Pulse nightclub incident follows another shooting in Orlando by a day. Singer Christina Grimmie, a 22-year-old YouTube star and one-time contestant of the TV talent show "The Voice," was fatally shot by a man outside her concert late Friday.

FILE - Christina Grimmie performs as the opener for Rachel Platten at Center Stage Theater, in Atlanta, March 2, 2016. Grimmie was shot by a gunman as she signed autographs for fans after a show, killing the onetime star of "The Voice" late Friday.